< BackNext >PreviewMain Plate Tectonics Section 1 Inside the EarthInside the Earth Section 2 Restless ContinentsRestless Continents Section 3 The Theory

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Plate Tectonics

Section 1 Inside the Earth

Section 2 Restless Continents

Section 3 The Theory of Plate Tectonics

Section 4 Deforming the Earth’s Crust

Chapter 7

Preview

Concept Mapping


Section 1 Inside the Earth

Bellringer

Chapter 7

If you journeyed to the center of the Earth, what do you think you would observe along the way? Draw an illustration of the journey in your science journal.


Section 1 Inside the EarthChapter 7

Objectives• Identify the layers of the Earth by their chemical composition.

• Identify the layers of the Earth by their physical properties.

• Describe a tectonic plate.

• Explain how scientists know about the structure of Earth’s interior.



The Composition of the Earth• The Earth is divided into three layers—the crust, the mantle, and the core—based on the compounds that make up each layer.

• The Crust is the outermost layer of the Earth. The crust is 5 to 100 km thick, and is the thinnest layer of the Earth.



The Composition of the Earth, continued

• There are two types of crust—continental and oceanic. Oceanic crust is thinner and denser than continental crust.




• The Mantle is the layer of the Earth between the crust and the core. The mantle is much thicker than the crust and contains most of the Earth’s mass.

• The crust is too thick to drill through, so scientists must draw conclusions about the composition and other properties of the mantle from observations made on the Earth’s surface.




• The Core is the central part of the Earth that lies below the mantle. The core makes up about one-third of Earth’s mass.

• Scientists think that the Earth’s core is made mostly of iron and contains smaller amounts of nickel but almost no oxygen, silicon, aluminum, or magnesium.





The Physical Structure of the Earth The Earth is divided into five physical layers:

• The lithosphere

• The asthenosphere

• The mesosphere

• The outer core

• The inner core

Each layer has its own set of physical properties.



Physical Structure of the Earth, continued

• The outermost, rigid layer of the Earth is called the lithosphere.

• The lithosphere is made of two parts—the crust and the rigid upper part of the mantle.

• The lithosphere is divided into pieces that are called tectonic plates.




• The asthenosphere is a plastic layer of the mantle on which the tectonic plates move.

• The asthenosphere is made of solid rock that flows very slowly.






• The mesosphere is the strong, lower part of the mantle between the asthenosphere and the outer core.

• The prefix meso- means “middle.”




• The Earth’s core is divided into two parts.

• The outer core is the liquid layer of the Earth’s core that lies beneath the mantle.

• The inner core is the solid, dense center of our planet that extends from the bottom of the outer core to the center of the Earth, about 6,380 km beneath the surface.





Tectonic Plates

• Pieces of the lithosphere that move around on top of the asthenosphere are called tectonic plates.

• Tectonic plates consist of the crust and the rigid, outermost part of the mantle.



Tectonic Plates, continued

• A Giant Jigsaw Puzzle Each tectonic plate fits together with the tectonic plates that surround it.

• The lithosphere is like a jigsaw puzzle. The tectonic plates are like the pieces of the puzzle.



Tectonic Plates, continued

• A Tectonic Plate Close-Up The following Visual Concept presentation shows the Earth’s major tectonic plates and how they fit together.

• The presentation also illustrates what a tectonic plate might look like if you could lift it out of its place.


Tectonic Plates

Click below to watch the Visual Concept.

Visual Concept




Tectonic Plates, continued• Tectonic plates “float” on the asthenosphere. The plates cover the surface of the asthenosphere, and they touch one another and move around.

• The lithosphere displaces the asthenosphere. Thick tectonic plates, such as those made of continental crust, displace more asthenosphere than do thin plates, such as those made of oceanic lithosphere.



Mapping the Earth’s Interior• Scientists have learned much about the deepest parts of the planet by measuring the speeds of the seismic waves that travel through the Earth’s interior during earthquakes.

• By using seismographs, scientists have learned that the Earth is made of different layers.


Seismographs and Mapping Earth’s Layers


Visual Concept



Section 2 Restless Continents

Bellringer

Chapter 7

What is meant by the statement: “The United States is moving westward”?

From what you know about geology and plate tectonics, explain if you believe this statement to be true or false.


Section 2 Restless ContinentsChapter 7

Objectives• Describe Wegener’s hypothesis of continental drift.

• Explain how sea-floor spreading provides a way for continents to move.

• Describe how new oceanic lithosphere forms at mid-ocean ridges.

• Explain how magnetic reversals provide evidence for sea-floor spreading.



Wegener’s Continental Drift Hypothesis

• Continental drift is the hypothesis that states that continents once formed a single landmass, broke up, and drifted to their present locations.

• Scientist Alfred Wegener developed the hypothesis in the early 1900s.



The Breakup of Pangaea• Wegener theorized that all of the present continents were once joined in a single, huge continent he called Pangaea.

• Pangaea is Greek for “all earth.”

• Pangaea existed about 245 million years ago.


Continental Drift


Visual Concept




Sea-Floor Spreading• Evidence to support the continental drift hypothesis comes from sea-floor spreading.

• Sea-floor spreading is the process by which new oceanic lithosphere forms as magma rises toward the surface and solidifies.



Sea-Floor Spreading, continued• Mid-Ocean Ridges and Sea-Floor Spreading Mid-ocean ridges are underwater mountain chains that run through Earth’s ocean basins.

• These mid-ocean ridges are the places where sea-floor spreading takes place.





Sea-Floor Spreading, continued• Evidence for Sea-Floor Spreading: Magnetic Reversals Some of the most important evidence of sea-floor spreading comes from magnetic reversals recorded in the ocean floor.

• Throughout Earth’s history, the north and south magnetic poles have changed places many times.



Sea-Floor Spreading, continued• Magnetic Reversals and Sea-Floor Spreading Molten rock at the mid-ocean ridge contains tiny grains of magnetic minerals that act like compasses.

• These minerals align with the magnetic field of the Earth. When the molten rock cools, the record of these tiny compasses remains in the rock.



Sea-Floor Spreading, continued• When the Earth’s magnetic field reverses, the magnetic mineral grains align in the opposite direction. The new rock records the direction of the Earth’s magnetic field.

• As the sea floor spreads away from a mid-ocean ridge, it carries with it a record of these magnetic reversals.


Magnetic Reversals and Sea-Floor Spreading


Visual Concept




Bellringer

Chapter 7

If the sea floor is spreading an average of 4 cm a year, how many years did it take New York and the northwest coast of Africa to reach their current locations, 676,000,000 cm apart?


Chapter 7

Objectives

• Describe the three types of tectonic plate boundaries.

• Describe the three forces thought to move tectonic plates.

• Explain how scientists measure the rate at which tectonic plates move.



Chapter 7

Tectonic Plate Boundaries• As scientists’ understanding of mid-ocean ridges and magnetic reversals grew, a theory was formed to explain how tectonic plates move.

• Plate tectonics is the theory that explains how large pieces of the Earth’s outermost layer, called tectonic plates, move and change shape.



Chapter 7

Tectonic Plate Boundaries, continued

• A boundary is a place where tectonic plates touch. All tectonic plates share boundaries with other tectonic plates.

• The type of boundary depends on how the tectonic plates move relative to one another.



Chapter 7


There are three types of tectonic plate boundaries:

• Convergent Boundaries

• Divergent Boundaries

• Transform Boundaries



Chapter 7

Tectonic Plate Boundaries, continued• When two tectonic plates collide, the boundary between them is a convergent boundary.

• What happens at convergent boundaries depends on the kind of crust at the leading edge of each tectonic plate.



Chapter 7 Section 3 The Theory of Plate Tectonics


Tectonic Plate Boundaries


Visual Concept



Chapter 7

• When two tectonic plates separate, the boundary between them is called a divergent boundary.

• New sea floor forms at divergent boundaries.




Chapter 7

• When two tectonic plates slide past each other horizontally, the boundary between is called a transform boundary.

• The San Andreas Fault in California is an example of a transform boundary.






Causes of Tectonic Plate Motion


Visual Concept



Chapter 7

Possible Causes of Tectonic Plate Motion

• What causes the motion of tectonic plates? This movement occurs because of changes in the density within the asthenosphere.

• The following Visual Concept presentation examines three possible driving forces of tectonic plate motion.



Chapter 7

Tracking Tectonic Plate Motion• Tectonic plate movements are so slow and gradual that you can’t see or feel them. The movement is measured in centimeters per year.

• Scientists use a system of satellites called the global positioning system (GPS) to measure the rate of tectonic plate movement.



Newton’s Second Law of Motion, continued


Visual Concept




Bellringer

Chapter 7

Compare the mountains in the photographs. Write a description of each mountain, and suggest how it might have formed.

Do you know where these various types of mountains are found in the world? Have you ever visited any of them? Would it ever be dangerous to study them?


Chapter 7

Objectives• Describe two types of stress that deform rocks.

• Describe three major types of folds.

• Explain the differences between the three major types of faults.

• Identify the most common types of mountains.

• Explain the difference between uplift and subsidence.



Chapter 7

Deformation• Whether a material bends or breaks depends on the how much stress is applied to the material.

• Stress is the amount of force per unit area on a given material.

• Different things happen to rock when different types of stress are applied.



Chapter 7

Deformation, continued• The process by which the shape of a rock changes because of stress is called deformation.

• Rock layers bend when stress is placed on them.

• When enough stress is placed on rocks, they can reach their elastic limit and break.



Chapter 7

Deformation, continued• The type of stress that occurs when an object is squeezed, such as when two tectonic plates collide, is called compression.

• When compression occurs at a convergent boundary, large mountain ranges can form.



Chapter 7

Deformation, continued• Tension is stress that occurs when forces act to stretch an object.

• Tension occurs at divergent plate boundaries, such as mid-ocean ridges, when two tectonic plates pull away from each other.



Chapter 7

Folding• The bending of rock layers because of stress in the Earth’s crust is called folding.

• Types of Folds Depending on how rock layers deform, different types of folds are made.

• The major types of folds are anticlines, synclines, and monoclines.



Chapter 7

Folding, continued• Anticlines are upward-arching folds.

• Synclines are downward, troughlike folds.



Chapter 7

Folding, continued• In a monocline, rock layers are folded so that both ends of the fold are horizontal.



Chapter 7

Faulting• Some rock layers break when stress is applied. The surface along which rocks break and slide past each other is called a fault.

• The blocks of crust on each side of the fault are called fault blocks.



Chapter 7

Faulting, continued• When a fault is not vertical, its two sides are either a hanging wall or a footwall.



Chapter 7

Faulting, continued• The type of fault depends on how the hanging wall and footwall move in relationship to each other.


•When a normal fault moves, it causes the hanging wall to move down relative to the footwall.


Chapter 7

Faulting, continued• When a reverse fault moves, it causes the hanging wall to move up relative to the footwall.



Chapter 7

Faulting, continued• A third major type of fault is a strike-slip fault. These faults form when opposing forces cause rock to break and move horizontally.



Chapter 7

Plate Tectonics and Mountain Building

• When tectonic plates collide, land features that start as folds and faults can eventually become large mountain ranges.

• When tectonic plates undergo compressions or tension, they can form mountains in several ways.



Chapter 7

Mountain Building, continued• Folded Mountains form when rock layers are squeezed together and pushed upward.

• Fault-Block Mountains form when large blocks of the Earth’s crust drop down relative to other blocks.

• Volcanic Mountains form when magma rises to the Earth’s surface and erupts.



Chapter 7

Uplift and Subsidence• Vertical movements in the crust are divided into two types—uplift and subsidence.

• Uplift is the rising of regions of the Earth’s crust to higher elevations.

• Subsidence is the sinking of regions of the Earth’s crust to lower elevations.



Chapter 7

Uplift and Subsidence, continued• Uplifting of Depressed Rocks Uplift can occur when large areas of land rise without deforming.

• One way areas rise without deforming is process known as rebound. When the crust rebounds, it slowly springs back to its previous elevation.



Chapter 7

Uplift and Subsidence, continued• Subsidence of Cooler Rocks Rocks that are hot take up more space than cooler rocks.

• The lithosphere is relatively hot at mid-ocean ridges, but cools as it moves farther from the ridge.

• As it cools, the oceanic lithosphere takes up less volume and the ocean floor subsides.



Chapter 7

Uplift and Subsidence, continued

• Tectonic Letdown Subsidence can also occur when the lithosphere becomes stretched in rift zones.

• A rift zone is a set of deep cracks that forms between two tectonic plates that are pulling away from each other.

• As tectonic plates pull apart, stress between the plates causes a series of faults to form along the rift zone.



Plate Tectonics

Concept MappingUse the terms below to complete the concept map on the next slide.

Chapter 7

transform boundaries

tectonic plates

divergent boundaries

converge

diverge


Plate TectonicsChapter 7


Plate TectonicsChapter 7

Documents

< BackNext >PreviewMain Plate Tectonics Section 1 Inside the EarthInside the Earth Section 2 Restless ContinentsRestless Continents Section 3 The Theory